1. Field of the Invention
This invention relates to a positive resist composition which lends itself to a lift-off technique.
2. Prior Art
In the prior art, dry and wet etching processes are commonly used in forming conductor patterns such as aluminum electrodes on semiconductor substrates. The process is illustrated in FIG. 3 as comprising metal layer sputtering and resist patterning to form a resist pattern 3 on a metallization layer 2 on a substrate 1. The exposed area of the metallization layer 2, that is, those metallization portions which are not covered with the resist pattern 3 are etched away. By treating the structure with a stripping solution, the resist pattern 3 is stripped off to leave a conductor pattern 2 on the substrate 1.
The above-mentioned process, however, suffers from low processing precision and a problem associated with etching procedure where difficult-to-etch metals such as gold and tantalum are used as the metallization. To avoid such inconvenience, a lift-off technique is often used in these years. The lift-off technique is illustrated in FIG. 4 as comprising resist patterning to form a resist pattern 3 on a substrate 1, metal sputtering to form a metal layer 2 on the resist pattern 3 and substrate 1, and processing the structure with a stripping solution to strip or lift off the resist. A conductor pattern can be formed without an etching procedure. This lift-off technique ensures precision processing since a metal pattern is formed using a precisely configured resist pattern as a template. Even a difficult-to-etch metal can be readily processed due to the eliminated need for etching as long as it can be deposited on the substrate and resist layer. The resist stripping step plays the most important role in the lift-off technique. If the resist can be effectively stripped in this step without leaving residues, the lift-off technique is expected to accomplish improvements in processing precision and reliability.
However, where patterns are formed by the lift-off technique using conventional positive resist compositions, the resist stripping step does not effectively take place and can leave resist residues, arising a problem of reliability.
Therefore, the lift-off technique often employs a step requiring complex, cumbersome operations, for example, a dual layer resist process and an image reversal resist process.
JP-A 69111/1996 (U.S. Ser. No. 573,578) discloses a resist material from which a pattern configuration suitable for the lift-off technique is obtained by a simple single-layer positive resist process. This resist material is epoch-making in that a pattern configuration suitable for the lift-off technique is obtained by an ordinary single-layer positive type one-step resist process while the resist material is prepared from three components, a novolak resin, a photosensitive agent, and a dissolution promoter. Then, in order to accomplish high resolution, dimensional control, heat resistance and film retention which are believed to be increasingly required in the future lift-off involving process, various contrivances concerning both the novolak resin and the photosensitive agent are necessary like conventional novolak base positive resist materials.
One exemplary measure associated with the novolak resin side is described. If a novolak resin is synthesized to a lower average molecular weight, the resulting resist layer is improved in resolution, but becomes poor in heat resistance and film retention. It then becomes necessary to take an appropriate measure of removing a low molecular weight novolak fraction as by subjecting the synthesized novolak resin to re-precipitation. The following measure is associated with the photosensitive agent side. As the light source of an aligner shifts from g-line to i-line, light is not easily transmitted by a resist layer containing a conventionally used benzophenone photosensitive agent. It then becomes necessary to take an appropriate measure of using a non-benzophenone photosensitive agent causing less absorption of i-line.
However, in order that a resist material for the lift-off technique having higher resolution as well as satisfactory dimensional control, heat resistance, and film retention be accomplished by combining such measures, many additional steps are necessary in the resist preparation process and undesirably cause a cost increase.
Therefore, an object of the present invention is to provide a positive resist composition for use in the lift-off technique which has solved the above-mentioned problems of the lift-off technique, which can form a reliable conductor pattern at high processing precision, which is sufficiently heat resistant to withstand the elevated temperature encountered upon metal sputtering, and which can be prepared by a simple process. Another object of the present invention is to provide a method for forming a wiring or conductor pattern using the resist composition according to the lift-off technique.
Although the lift-off technique generally needs complex steps using a dual-layer resist or image reversal resist, the invention intends to provide a positive resist composition from which a resist pattern having high working precision and reliability and amenable to the lift-off technique can be formed through a single-layer positive type one-step resist process. The resultant resist pattern has sufficient heat resistance to withstand the heat encountered upon metal sputtering, can be formed by simple steps, and can be advantageously used in forming a wiring pattern such as an electrode of a difficult-to-etch metal.